Chen and Frangogiannis Macrophages in Heart Failure 777 adhesion cascades that trigger leukocyte extravasation. In contrast, the mature scar is characterized by a low capillary density and a relative abundance of mature neovessels coated with mural cells, 19 hampering leukocyte extravasation. Moreover, the cross-linked extracellular matrix in the mature scar may prevent monocyte migration and may deactivate macrophages, thus attenuating macrophage expansion. Because of its low cellular content, the mature scar may not be able to generate gradients of chemotactic mediators, necessary for monocyte recruitment.